diff --git a/codex-rs/app-server-protocol/schema/precomputed/app-server-exports-experimental.json.zst b/codex-rs/app-server-protocol/schema/precomputed/app-server-exports-experimental.json.zst
index 5e3d6b67ce..708b89be3b 100644
Binary files a/codex-rs/app-server-protocol/schema/precomputed/app-server-exports-experimental.json.zst and b/codex-rs/app-server-protocol/schema/precomputed/app-server-exports-experimental.json.zst differ
diff --git a/codex-rs/app-server-protocol/src/protocol/common.rs b/codex-rs/app-server-protocol/src/protocol/common.rs
index 8d5d2be81d..1afa3c6b42 100644
--- a/codex-rs/app-server-protocol/src/protocol/common.rs
+++ b/codex-rs/app-server-protocol/src/protocol/common.rs
@@ -1338,6 +1338,13 @@ client_request_definitions! {
         response: v2::BedrockSetupResponse,
     },
 
+    #[experimental("account/gcp/setup")]
+    GcpSetup => "account/gcp/setup" {
+        params: v2::GcpSetupParams,
+        serialization: global("account-auth"),
+        response: v2::GcpSetupResponse,
+    },
+
     #[experimental("account/bedrock/checkGovCloudRequirements")]
     BedrockCheckGovCloudRequirements => "account/bedrock/checkGovCloudRequirements" {
         params: v2::BedrockCheckGovCloudRequirementsParams,
diff --git a/codex-rs/app-server-protocol/src/protocol/v2/gcp.rs b/codex-rs/app-server-protocol/src/protocol/v2/gcp.rs
new file mode 100644
index 0000000000..97b18ef66c
--- /dev/null
+++ b/codex-rs/app-server-protocol/src/protocol/v2/gcp.rs
@@ -0,0 +1,27 @@
+//! Shared setup parameters for the built-in Google Cloud Vertex AI provider.
+
+use crate::JsonSchema;
+use crate::TS;
+use serde::Deserialize;
+use serde::Serialize;
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct GcpSetupParams {
+    pub project_id: String,
+    /// Vertex AI location. Omission or null selects `global`.
+    #[ts(optional = nullable)]
+    pub location: Option<String>,
+    /// Absolute path to a credential file on the app-server host.
+    /// Omit or pass `null` to clear the active user configuration's override and use ADC.
+    /// If another configuration layer would still supply a credential file, setup fails
+    /// without saving changes.
+    #[ts(optional = nullable)]
+    pub credential_file: Option<String>,
+}
+
+#[derive(Serialize, Deserialize, Debug, Default, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct GcpSetupResponse {}
diff --git a/codex-rs/app-server-protocol/src/protocol/v2/mod.rs b/codex-rs/app-server-protocol/src/protocol/v2/mod.rs
index f2f157fe7f..37b6f5ad20 100644
--- a/codex-rs/app-server-protocol/src/protocol/v2/mod.rs
+++ b/codex-rs/app-server-protocol/src/protocol/v2/mod.rs
@@ -16,6 +16,7 @@ mod environment;
 mod experimental_feature;
 mod feedback;
 mod fs;
+mod gcp;
 mod hook;
 mod item;
 mod mcp;
@@ -57,6 +58,7 @@ pub use environment::*;
 pub use experimental_feature::*;
 pub use feedback::*;
 pub use fs::*;
+pub use gcp::*;
 pub use hook::*;
 pub use item::*;
 pub use mcp::*;
diff --git a/codex-rs/app-server/README.md b/codex-rs/app-server/README.md
index 93533fce60..26a385cec5 100644
--- a/codex-rs/app-server/README.md
+++ b/codex-rs/app-server/README.md
@@ -646,3 +646,36 @@ and out-of-scope item IDs return invalid params (`-32602`) with
 Omitted or null cursors preserve normal first-page behavior. Continue anchored
 pages with the returned opaque string `nextCursor`; response fields and
 `backwardsCursor` semantics are unchanged.
+
+# GCP provider account
+
+The experimental `account/gcp/setup` method configures the built-in Vertex AI provider.
+
+- `projectId` (required) — Google Cloud project ID.
+- `location` (optional) — Defaults to `global`.
+- `credentialFile` (optional) — Absolute path to a credential file on the app-server
+  host. Omit it or pass `null` to clear the saved path in the active user
+  configuration and use Application Default Credentials (ADC), including
+  `GOOGLE_APPLICATION_CREDENTIALS`.
+
+If the active profile inherits `model_providers.gcp.credential_file` from the base
+`config.toml`, clearing the profile’s own setting would expose the base path again.
+Setup returns an error without saving changes. To use ADC, remove the inherited
+setting from the file named in the error and retry.
+
+Setup rejects a GCP `base_url` override or higher-priority settings that conflict
+with the request. It fetches an access token before saving the settings and
+selecting GCP, then returns `{}`. The next `model/list` reloads the catalog.
+`account/read` reports `{ "account": { "type": "gcp" }, "requiresOpenaiAuth": false }`.
+Use a model request to verify Vertex model access.
+
+Credential failures include `error.data.reason`.
+
+- `gcloudNotInstalled` — The app-server could not find `gcloud` on its PATH.
+- `credentialFileInvalid` — The path is invalid or the file is unreadable. For
+  default `gcloud` auth, setup also checks for a JSON object with a nonempty string `type`.
+- `authCommandFailed` — The token command failed, including when ADC is missing
+  or a token refresh fails.
+
+For personal sign-in, users can run `gcloud auth application-default login` on
+the app-server host and retry setup.
diff --git a/codex-rs/app-server/src/config_manager_service.rs b/codex-rs/app-server/src/config_manager_service.rs
index df807ba4b5..ebb3b5efc5 100644
--- a/codex-rs/app-server/src/config_manager_service.rs
+++ b/codex-rs/app-server/src/config_manager_service.rs
@@ -3,6 +3,7 @@ use crate::config_layer::config_layer_to_api;
 use crate::config_manager::ConfigManager;
 use codex_app_server_protocol::Config as ApiConfig;
 use codex_app_server_protocol::ConfigBatchWriteParams;
+use codex_app_server_protocol::ConfigEdit as ApiConfigEdit;
 use codex_app_server_protocol::ConfigReadParams;
 use codex_app_server_protocol::ConfigReadResponse;
 use codex_app_server_protocol::ConfigValueWriteParams;
@@ -31,6 +32,7 @@ use codex_core::path_utils;
 use codex_core::path_utils::SymlinkWritePaths;
 use codex_core::path_utils::resolve_symlink_write_paths;
 use codex_core::path_utils::write_atomically;
+use codex_model_provider_info::ModelProviderInfo;
 use codex_protocol::protocol::AskForApproval;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use serde_json::Value as JsonValue;
@@ -44,6 +46,8 @@ use toml_edit::Item as TomlItem;
 
 #[path = "config_manager_service_credential_edits.rs"]
 mod credential_edits;
+#[path = "config_manager_service_provider_setup.rs"]
+mod provider_setup;
 
 use credential_edits::CredentialProviderEdits;
 
@@ -116,6 +120,11 @@ impl ConfigManagerError {
     }
 }
 
+pub(crate) struct PreparedProviderWrite {
+    edits: Vec<ApiConfigEdit>,
+    expected_version: String,
+}
+
 impl ConfigManager {
     pub(crate) async fn read(
         &self,
@@ -202,6 +211,63 @@ impl ConfigManager {
             .await
     }
 
+    pub(crate) fn prepare_provider_config<'a>(
+        &self,
+        layers: &ConfigLayerStack,
+        provider_id: &'a str,
+        settings: impl IntoIterator<Item = (&'a str, Option<&'a str>)>,
+    ) -> Result<(PreparedProviderWrite, ModelProviderInfo), ConfigManagerError> {
+        let selection = match layers.required_model_provider() {
+            Some(required) if required == provider_id => None,
+            Some(required) => {
+                return Err(ConfigManagerError::write(
+                    ConfigWriteErrorCode::ConfigRequirementReadonly,
+                    format!(
+                        "Cannot select `{provider_id}` because requirements set `model_provider` to `{required}`."
+                    ),
+                ));
+            }
+            None => Some(("model_provider".to_owned(), Some(provider_id))),
+        };
+        let settings = settings
+            .into_iter()
+            .map(|(key, value)| (format!("model_providers.{provider_id}.{key}"), value));
+        let edits = selection
+            .into_iter()
+            .chain(settings)
+            .map(|(key_path, value)| ApiConfigEdit {
+                key_path,
+                value: value.into(),
+                merge_strategy: MergeStrategy::Replace,
+            })
+            .collect();
+        let user_path = self
+            .user_config_path()
+            .map_err(|err| ConfigManagerError::io("failed to resolve user config path", err))?;
+        provider_setup::preview(layers, &user_path, provider_id, edits)
+    }
+
+    pub(crate) async fn commit_provider_config(
+        &self,
+        write: PreparedProviderWrite,
+    ) -> Result<(), ConfigManagerError> {
+        let response = self
+            .batch_write(ConfigBatchWriteParams {
+                edits: write.edits,
+                file_path: None,
+                expected_version: Some(write.expected_version),
+                reload_user_config: false,
+            })
+            .await?;
+        if let Some(overridden) = response.overridden_metadata {
+            return Err(ConfigManagerError::write(
+                ConfigWriteErrorCode::ConfigLayerReadonly,
+                overridden.message,
+            ));
+        }
+        Ok(())
+    }
+
     pub(crate) async fn batch_write(
         &self,
         params: ConfigBatchWriteParams,
@@ -262,18 +328,7 @@ impl ConfigManager {
         let mut config_edits = Vec::new();
 
         for (key_path, value, strategy) in edits.into_iter() {
-            let mut segments = parse_key_path(&key_path).map_err(|message| {
-                ConfigManagerError::write(ConfigWriteErrorCode::ConfigValidationError, message)
-            })?;
-            if let Some(field) = layers
-                .requirements_toml()
-                .exact_requirement_for_config_path(&segments)
-            {
-                return Err(ConfigManagerError::write(
-                    ConfigWriteErrorCode::ConfigRequirementReadonly,
-                    format!("`{field}` is managed by requirements and cannot be changed"),
-                ));
-            }
+            let mut segments = writable_key_path(&layers, &key_path)?;
             if (value.is_null() || matches!(strategy, MergeStrategy::Upsert))
                 && let Some(pattern) = shell_environment_filter_entry(&user_config, &segments)
                     .map(|(pattern, _)| pattern.clone())
@@ -318,9 +373,7 @@ impl ConfigManager {
             let parsed_value = if preserves_network_proxy_settings {
                 Some(TomlValue::Boolean(false))
             } else {
-                parse_value(value).map_err(|message| {
-                    ConfigManagerError::write(ConfigWriteErrorCode::ConfigValidationError, message)
-                })?
+                parse_value(value)?
             };
             if matches!(strategy, MergeStrategy::Upsert)
                 && let Some(value) = parsed_value.as_ref()
@@ -526,14 +579,38 @@ async fn write_empty_user_config(write_path: PathBuf) -> Result<(), ConfigManage
         .map_err(|err| ConfigManagerError::io("failed to create empty user config.toml", err))
 }
 
-fn parse_value(value: JsonValue) -> Result<Option<TomlValue>, String> {
+fn parse_value(value: JsonValue) -> Result<Option<TomlValue>, ConfigManagerError> {
     if value.is_null() {
         return Ok(None);
     }
 
     serde_json::from_value::<TomlValue>(value)
         .map(Some)
-        .map_err(|err| format!("invalid value: {err}"))
+        .map_err(|err| {
+            ConfigManagerError::write(
+                ConfigWriteErrorCode::ConfigValidationError,
+                format!("invalid value: {err}"),
+            )
+        })
+}
+
+fn writable_key_path(
+    layers: &ConfigLayerStack,
+    key_path: &str,
+) -> Result<Vec<String>, ConfigManagerError> {
+    let segments = parse_key_path(key_path).map_err(|message| {
+        ConfigManagerError::write(ConfigWriteErrorCode::ConfigValidationError, message)
+    })?;
+    if let Some(field) = layers
+        .requirements_toml()
+        .exact_requirement_for_config_path(&segments)
+    {
+        return Err(ConfigManagerError::write(
+            ConfigWriteErrorCode::ConfigRequirementReadonly,
+            format!("`{field}` is managed by requirements and cannot be changed"),
+        ));
+    }
+    Ok(segments)
 }
 
 fn parse_key_path(path: &str) -> Result<Vec<String>, String> {
diff --git a/codex-rs/app-server/src/config_manager_service_provider_setup.rs b/codex-rs/app-server/src/config_manager_service_provider_setup.rs
new file mode 100644
index 0000000000..d66114663f
--- /dev/null
+++ b/codex-rs/app-server/src/config_manager_service_provider_setup.rs
@@ -0,0 +1,87 @@
+//! Previews the effective provider configuration and captures the user layer version for commit.
+
+use super::ConfigManagerError;
+use super::MergeError;
+use super::PreparedProviderWrite;
+use super::apply_merge;
+use super::find_effective_layer;
+use super::override_message;
+use super::parse_value;
+use super::value_at_path;
+use super::writable_key_path;
+use codex_app_server_protocol::ConfigEdit;
+use codex_app_server_protocol::ConfigWriteErrorCode;
+use codex_config::ConfigLayerStack;
+use codex_config::version_for_toml;
+use codex_model_provider_info::ModelProviderInfo;
+use codex_utils_absolute_path::AbsolutePathBuf;
+use toml::Value as TomlValue;
+
+pub(super) fn preview(
+    layers: &ConfigLayerStack,
+    user_path: &AbsolutePathBuf,
+    provider_id: &str,
+    edits: Vec<ConfigEdit>,
+) -> Result<(PreparedProviderWrite, ModelProviderInfo), ConfigManagerError> {
+    let (mut user_config, expected_version) = match layers.get_active_user_layer() {
+        Some(layer) => (layer.config.clone(), layer.version.clone()),
+        None => {
+            let empty = TomlValue::Table(Default::default());
+            let version = version_for_toml(&empty);
+            (empty, version)
+        }
+    };
+    let mut fields = Vec::with_capacity(edits.len());
+    for edit in &edits {
+        let path = writable_key_path(layers, &edit.key_path)?;
+        let value = parse_value(edit.value.clone())?;
+        apply_merge(
+            &mut user_config,
+            &path,
+            value.as_ref(),
+            edit.merge_strategy.clone(),
+        )
+        .map_err(|MergeError::Validation(message)| {
+            ConfigManagerError::write(ConfigWriteErrorCode::ConfigValidationError, message)
+        })?;
+        fields.push((&edit.key_path, path, value));
+    }
+
+    let preview = layers
+        .with_user_config(user_path, user_config)
+        .map_err(|err| {
+            ConfigManagerError::write(ConfigWriteErrorCode::ConfigValidationError, err.to_string())
+        })?;
+    let effective = preview.effective_config();
+    for (key, path, expected) in fields {
+        if value_at_path(&effective, &path) != expected.as_ref() {
+            let source = find_effective_layer(&preview, &path)
+                .map(|layer| override_message(&layer.name))
+                .unwrap_or_else(|| "Overridden by another configuration layer".to_owned());
+            return Err(ConfigManagerError::write(
+                ConfigWriteErrorCode::ConfigLayerReadonly,
+                format!("Cannot configure `{key}`: {source}"),
+            ));
+        }
+    }
+    let provider = effective
+        .get("model_providers")
+        .and_then(|providers| providers.get(provider_id))
+        .cloned()
+        .map(TomlValue::try_into)
+        .transpose()
+        .map_err(|_| {
+            ConfigManagerError::write(
+                ConfigWriteErrorCode::ConfigValidationError,
+                format!("Invalid `{provider_id}` provider configuration."),
+            )
+        })?
+        .unwrap_or_default();
+    Ok((
+        PreparedProviderWrite {
+            edits,
+            expected_version,
+        },
+        provider,
+    ))
+}
diff --git a/codex-rs/app-server/src/config_manager_service_tests.rs b/codex-rs/app-server/src/config_manager_service_tests.rs
index dad7956ee1..2948434c95 100644
--- a/codex-rs/app-server/src/config_manager_service_tests.rs
+++ b/codex-rs/app-server/src/config_manager_service_tests.rs
@@ -1187,6 +1187,29 @@ async fn write_value_reports_override() {
     assert!(result.overridden_metadata.is_none());
 }
 
+#[tokio::test]
+async fn provider_setup_rejects_config_changed_after_preview() -> Result<()> {
+    let tmp = tempdir()?;
+    let service = ConfigManager::without_managed_config_for_tests(tmp.path().to_path_buf());
+    let layers = service.load_config_layers(/*cwd*/ None).await?;
+    let (write, _) = service.prepare_provider_config(
+        &layers,
+        codex_model_provider_info::AMAZON_BEDROCK_PROVIDER_ID,
+        [("aws.region", Some("us-west-2"))],
+    )?;
+
+    let user_path = tmp.path().join(CONFIG_TOML_FILE);
+    let newer_config = "model = 'newer'";
+    std::fs::write(&user_path, newer_config)?;
+    let error = service.commit_provider_config(write).await.unwrap_err();
+    assert_eq!(
+        error.write_error_code(),
+        Some(ConfigWriteErrorCode::ConfigVersionConflict)
+    );
+    assert_eq!(std::fs::read_to_string(user_path)?, newer_config);
+    Ok(())
+}
+
 #[tokio::test]
 async fn version_conflict_rejected() {
     let tmp = tempdir().expect("tempdir");
diff --git a/codex-rs/app-server/src/message_processor.rs b/codex-rs/app-server/src/message_processor.rs
index a1bb6d1901..c664fac4b2 100644
--- a/codex-rs/app-server/src/message_processor.rs
+++ b/codex-rs/app-server/src/message_processor.rs
@@ -50,6 +50,7 @@ use crate::request_processors::ThreadRequestProcessor;
 use crate::request_processors::ThreadResumeTarget;
 use crate::request_processors::TurnRequestProcessor;
 use crate::request_processors::WindowsSandboxRequestProcessor;
+use crate::request_processors::account_processor::handle_gcp_setup;
 use crate::request_processors::read_server_diagnostics;
 use crate::request_serialization::QueuedInitializedRequest;
 use crate::request_serialization::RequestSerializationQueueKey;
@@ -1806,6 +1807,9 @@ impl MessageProcessor {
             ClientRequest::BedrockSetup { params, .. } => {
                 self.account_processor.bedrock_setup(params).await
             }
+            ClientRequest::GcpSetup { params, .. } => {
+                handle_gcp_setup(&self.account_processor, params).await
+            }
             ClientRequest::BedrockCheckGovCloudRequirements { .. } => {
                 self.account_processor
                     .bedrock_check_gov_cloud_requirements()
diff --git a/codex-rs/app-server/src/model_catalog.rs b/codex-rs/app-server/src/model_catalog.rs
index ff9704fa30..8413c3d530 100644
--- a/codex-rs/app-server/src/model_catalog.rs
+++ b/codex-rs/app-server/src/model_catalog.rs
@@ -3,12 +3,14 @@
 use std::path::PathBuf;
 use std::sync::Arc;
 
+use codex_config::ConfigLayerStack;
 use codex_core::config::Config;
 use codex_features::Feature;
 use codex_login::AuthManager;
 use codex_models_manager::manager::RefreshStrategy;
 use codex_models_manager::manager::SharedModelsManager;
 use codex_protocol::openai_models::ModelPreset;
+use codex_utils_absolute_path::AbsolutePathBuf;
 use tokio::sync::Mutex;
 
 use crate::config_manager::ConfigManager;
@@ -48,6 +50,16 @@ impl ModelCatalog {
         }
     }
 
+    /// Loads the same startup context used to reload the catalog after account setup.
+    pub(crate) async fn load_startup_config_layers(&self) -> std::io::Result<ConfigLayerStack> {
+        let cwd = self
+            .startup_cwd
+            .as_deref()
+            .map(AbsolutePathBuf::from_absolute_path)
+            .transpose()?;
+        self.config_manager.load_config_layers(cwd).await
+    }
+
     /// Marks catalog configuration stale after an account transition.
     pub(crate) async fn invalidate(&self) {
         self.state.lock().await.needs_reload = true;
diff --git a/codex-rs/app-server/src/request_processors/account_processor.rs b/codex-rs/app-server/src/request_processors/account_processor.rs
index 43946dc1ed..2aaa283f78 100644
--- a/codex-rs/app-server/src/request_processors/account_processor.rs
+++ b/codex-rs/app-server/src/request_processors/account_processor.rs
@@ -1,7 +1,7 @@
 use super::bedrock_auth::BedrockProviderConfig;
 use super::bedrock_auth::clear_user_model_provider_if_bedrock;
-use super::bedrock_auth::configure_bedrock_provider;
-use super::bedrock_auth::ensure_user_model_provider_can_be_bedrock;
+use super::bedrock_auth::prepare_bedrock_provider;
+use super::config_processor::map_error as map_config_error;
 use super::*;
 use crate::auth_mode::auth_mode_to_api;
 use crate::external_auth::ExternalAuthBridge;
@@ -21,11 +21,13 @@ mod bedrock_gov_cloud;
 mod bedrock_setup;
 mod enterprise_login;
 mod gateway_oauth;
+mod gcp_setup;
 mod rate_limit_resets;
 mod workspace_routing;
 
 pub(super) use enterprise_login::EnterpriseLoginCompletion;
 pub(super) use enterprise_login::EnterpriseLoginTarget;
+pub(crate) use gcp_setup::handle_gcp_setup;
 
 // Duration before a browser ChatGPT login attempt is abandoned.
 const LOGIN_CHATGPT_TIMEOUT: Duration = Duration::from_secs(10 * 60);
@@ -80,6 +82,11 @@ enum RefreshTokenRequestOutcome {
     FailedPermanently,
 }
 
+enum CloudLoginProvider {
+    AmazonBedrock,
+    Gcp,
+}
+
 enum BedrockLoginCredentials {
     ApiKey(String),
     AccessKeys {
@@ -426,7 +433,10 @@ impl AccountRequestProcessor {
         )
     }
 
-    fn ensure_bedrock_login_allowed(&self) -> Result<(), JSONRPCErrorError> {
+    fn ensure_provider_login_allowed(
+        &self,
+        provider: CloudLoginProvider,
+    ) -> Result<(), JSONRPCErrorError> {
         if self.auth_manager.is_workload_identity_selected() {
             return Err(self.configured_auth_owned_by_host_error());
         }
@@ -437,9 +447,13 @@ impl AccountRequestProcessor {
             .auth_manager
             .is_login_method_allowed(ForcedLoginMethod::Api)
         {
-            return Err(invalid_request(
-                "Amazon Bedrock login is disabled. Use ChatGPT login instead.",
-            ));
+            let operation = match provider {
+                CloudLoginProvider::AmazonBedrock => "Amazon Bedrock login",
+                CloudLoginProvider::Gcp => "Google Cloud setup",
+            };
+            return Err(invalid_request(format!(
+                "{operation} is disabled. Use ChatGPT login instead."
+            )));
         }
         Ok(())
     }
@@ -508,7 +522,7 @@ impl AccountRequestProcessor {
         region: String,
     ) {
         let result = async {
-            self.ensure_bedrock_login_allowed()?;
+            self.ensure_provider_login_allowed(CloudLoginProvider::AmazonBedrock)?;
 
             match &credentials {
                 BedrockLoginCredentials::ApiKey(api_key) => {
@@ -536,8 +550,7 @@ impl AccountRequestProcessor {
             }
 
             self.cancel_active_login().await;
-            ensure_user_model_provider_can_be_bedrock(&self.config_manager).await?;
-            configure_bedrock_provider(
+            let write = prepare_bedrock_provider(
                 &self.config_manager,
                 BedrockProviderConfig {
                     region: matches!(&credentials, BedrockLoginCredentials::AccessKeys { .. })
@@ -546,6 +559,10 @@ impl AccountRequestProcessor {
                 },
             )
             .await?;
+            self.config_manager
+                .commit_provider_config(write)
+                .await
+                .map_err(map_config_error)?;
 
             match credentials {
                 BedrockLoginCredentials::ApiKey(api_key) => login_with_bedrock_api_key(
diff --git a/codex-rs/app-server/src/request_processors/account_processor/bedrock_setup.rs b/codex-rs/app-server/src/request_processors/account_processor/bedrock_setup.rs
index a3fe8f9108..aa609e73dd 100644
--- a/codex-rs/app-server/src/request_processors/account_processor/bedrock_setup.rs
+++ b/codex-rs/app-server/src/request_processors/account_processor/bedrock_setup.rs
@@ -1,7 +1,8 @@
 use super::super::bedrock_auth::BedrockProviderConfig;
-use super::super::bedrock_auth::configure_bedrock_provider;
-use super::super::bedrock_auth::ensure_user_model_provider_can_be_bedrock;
+use super::super::bedrock_auth::prepare_bedrock_provider;
+use super::super::config_processor::map_error as map_config_error;
 use super::AccountRequestProcessor;
+use super::CloudLoginProvider;
 use crate::error_code::internal_error;
 use crate::error_code::invalid_request;
 use codex_app_server_protocol::AwsCredentialType;
@@ -25,7 +26,7 @@ impl AccountRequestProcessor {
         &self,
         _params: BedrockDiscoverParams,
     ) -> Result<Option<ClientResponsePayload>, JSONRPCErrorError> {
-        self.ensure_bedrock_login_allowed()?;
+        self.ensure_provider_login_allowed(CloudLoginProvider::AmazonBedrock)?;
         let profiles = codex_aws_auth::discover_aws_profiles()
             .await
             .map_err(|err| internal_error(format!("failed to discover AWS profiles: {err}")))?
@@ -69,9 +70,22 @@ impl AccountRequestProcessor {
         &self,
         params: BedrockSetupParams,
     ) -> Result<Option<ClientResponsePayload>, JSONRPCErrorError> {
-        self.ensure_bedrock_login_allowed()?;
-        ensure_user_model_provider_can_be_bedrock(&self.config_manager).await?;
-        if matches!(&params, BedrockSetupParams::Environment { .. })
+        self.ensure_provider_login_allowed(CloudLoginProvider::AmazonBedrock)?;
+        let (profile, region) = match &params {
+            BedrockSetupParams::Profile { profile, region } => {
+                (Some(profile.trim()), region.trim())
+            }
+            BedrockSetupParams::Environment { region } => (None, region.trim()),
+        };
+        let write = prepare_bedrock_provider(
+            &self.config_manager,
+            BedrockProviderConfig {
+                region: Some(region),
+                profile,
+            },
+        )
+        .await?;
+        if profile.is_none()
             && matches!(
                 self.auth_manager.auth_cached(),
                 Some(CodexAuth::BedrockApiKey(_) | CodexAuth::BedrockAccessKeys(_))
@@ -82,58 +96,44 @@ impl AccountRequestProcessor {
             ));
         }
 
-        let region = match &params {
-            BedrockSetupParams::Profile { region, .. }
-            | BedrockSetupParams::Environment { region, .. } => region.trim(),
-        };
         if !is_supported_amazon_bedrock_region(region) {
             return Err(invalid_request(format!(
                 "Amazon Bedrock does not support region `{region}`"
             )));
         }
 
-        let profile = match &params {
-            BedrockSetupParams::Profile { profile, .. } => {
-                let profile = profile.trim();
-                if profile.is_empty() {
-                    return Err(invalid_request("AWS profile name must not be empty."));
-                }
-                codex_aws_auth::validate_aws_profile(
-                    profile,
-                    region,
-                    self.config.http_client_factory(),
-                )
-                .await
-                .map_err(|err| {
-                    invalid_request(format!(
-                        "failed to load credentials for AWS profile `{profile}`: {err}"
-                    ))
-                })?;
-                Some(profile.to_string())
+        if let Some(profile) = profile {
+            if profile.is_empty() {
+                return Err(invalid_request("AWS profile name must not be empty."));
             }
-            BedrockSetupParams::Environment { .. } => {
-                let has_bedrock_api_key = non_empty_env_var(AWS_BEARER_TOKEN_BEDROCK).is_some();
-                let has_access_keys = non_empty_env_var(AWS_ACCESS_KEY_ID).is_some()
-                    && non_empty_env_var(AWS_SECRET_ACCESS_KEY).is_some();
-                if !has_bedrock_api_key && !has_access_keys {
-                    return Err(invalid_request(
-                        "No AWS credentials found. Please Configure AWS credentials or complete AWS sign-in, then try again.",
-                    ));
-                }
-                None
+            codex_aws_auth::validate_aws_profile(
+                profile,
+                region,
+                self.config.http_client_factory(),
+            )
+            .await
+            .map_err(|err| {
+                invalid_request(format!(
+                    "failed to load credentials for AWS profile `{profile}`: {err}"
+                ))
+            })?;
+        } else {
+            let has_bedrock_api_key = non_empty_env_var(AWS_BEARER_TOKEN_BEDROCK).is_some();
+            let has_access_keys = non_empty_env_var(AWS_ACCESS_KEY_ID).is_some()
+                && non_empty_env_var(AWS_SECRET_ACCESS_KEY).is_some();
+            if !has_bedrock_api_key && !has_access_keys {
+                return Err(invalid_request(
+                    "No AWS credentials found. Please Configure AWS credentials or complete AWS sign-in, then try again.",
+                ));
             }
-        };
+        }
 
         // TODO: Validate the selected credentials against Bedrock ListModels once supported.
         self.cancel_active_login().await;
-        configure_bedrock_provider(
-            &self.config_manager,
-            BedrockProviderConfig {
-                region: Some(region),
-                profile: profile.as_deref(),
-            },
-        )
-        .await?;
+        self.config_manager
+            .commit_provider_config(write)
+            .await
+            .map_err(map_config_error)?;
 
         Ok(Some(BedrockSetupResponse {}.into()))
     }
diff --git a/codex-rs/app-server/src/request_processors/account_processor/gcp_setup.rs b/codex-rs/app-server/src/request_processors/account_processor/gcp_setup.rs
new file mode 100644
index 0000000000..c7989eb5a1
--- /dev/null
+++ b/codex-rs/app-server/src/request_processors/account_processor/gcp_setup.rs
@@ -0,0 +1,207 @@
+//! Validates and persists GCP setup without changing saved login credentials or existing threads.
+
+use super::super::config_processor::map_error as map_config_error;
+use super::AccountRequestProcessor;
+use super::CloudLoginProvider;
+use crate::error_code::internal_error;
+use crate::error_code::invalid_request;
+use codex_app_server_protocol::ClientResponsePayload;
+use codex_app_server_protocol::GcpSetupParams;
+use codex_app_server_protocol::GcpSetupResponse;
+use codex_app_server_protocol::JSONRPCErrorError;
+use codex_config::apply_default_gcp_auth;
+use codex_login::RefreshTokenError;
+use codex_model_provider::create_model_provider;
+use codex_model_provider_info::GCP_PROVIDER_ID;
+use codex_model_provider_info::ModelProviderInfo;
+use codex_model_provider_info::merge_gcp_provider_overrides;
+use codex_utils_absolute_path::AbsolutePathBuf;
+use serde::Deserialize;
+use serde::Deserializer;
+use serde::Serialize;
+use serde::de::MapAccess;
+use serde::de::Visitor;
+use serde::de::value::MapAccessDeserializer;
+use serde_json::json;
+use std::fmt;
+use std::io::BufReader;
+use std::io::ErrorKind;
+use std::path::Path;
+use std::path::PathBuf;
+use tokio::fs::File;
+
+pub(crate) async fn handle_gcp_setup(
+    processor: &AccountRequestProcessor,
+    params: GcpSetupParams,
+) -> Result<Option<ClientResponsePayload>, JSONRPCErrorError> {
+    processor.ensure_provider_login_allowed(CloudLoginProvider::Gcp)?;
+    let network_policy = processor
+        .auth_manager
+        .application_network_policy()
+        .for_current_account();
+    let layers = processor
+        .model_catalog
+        .load_startup_config_layers()
+        .await
+        .map_err(|err| internal_error(format!("failed to load configuration layers: {err}")))?;
+    let (write, provider) = processor
+        .config_manager
+        .prepare_provider_config(
+            &layers,
+            GCP_PROVIDER_ID,
+            [
+                ("project_id", Some(params.project_id.trim())),
+                (
+                    "location",
+                    Some(params.location.as_deref().unwrap_or("global").trim()),
+                ),
+                ("credential_file", params.credential_file.as_deref()),
+            ],
+        )
+        .map_err(map_config_error)?;
+    if provider.base_url.is_some() {
+        return Err(invalid_request(
+            "Google Cloud has a custom `base_url`. Clear `model_providers.gcp.base_url` before configuring a project and location.",
+        ));
+    }
+    // Even checking a Windows UNC credential path can contact the remote host.
+    let permit = network_policy
+        .acquire_for_unsupported_sdk()
+        .map_err(|err| invalid_request(err.to_string()))?;
+    permit
+        .run(verify_credentials(provider, &processor.config.cwd))
+        .await
+        .map_err(|err| invalid_request(err.to_string()))??;
+
+    processor
+        .config_manager
+        .commit_provider_config(write)
+        .await
+        .map_err(map_config_error)?;
+    processor.cancel_active_login().await;
+    processor.model_catalog.invalidate().await;
+    Ok(Some(GcpSetupResponse {}.into()))
+}
+
+#[derive(Serialize)]
+#[serde(rename_all = "camelCase")]
+enum AuthErrorReason {
+    GcloudNotInstalled,
+    CredentialFileInvalid,
+    AuthCommandFailed,
+}
+
+impl AuthErrorReason {
+    fn error(self, message: impl Into<String>) -> JSONRPCErrorError {
+        let mut error = invalid_request(message);
+        error.data = Some(json!({ "reason": self }));
+        error
+    }
+}
+
+async fn open_credential_file(path: &Path) -> std::io::Result<File> {
+    if !tokio::fs::metadata(path).await?.is_file() {
+        return Err(std::io::Error::new(
+            ErrorKind::InvalidInput,
+            "expected a regular file",
+        ));
+    }
+    File::open(path).await
+}
+
+// Require a JSON object. Serde also accepts positional arrays when deserializing a struct.
+struct AdcObject;
+
+impl<'de> Visitor<'de> for AdcObject {
+    type Value = ();
+
+    fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        formatter.write_str("a Google Cloud credential object")
+    }
+
+    fn visit_map<A: MapAccess<'de>>(self, map: A) -> Result<(), A::Error> {
+        #[derive(Deserialize)]
+        struct Adc {
+            #[serde(rename = "type")]
+            kind: String,
+        }
+        let adc = Adc::deserialize(MapAccessDeserializer::new(map))?;
+        if adc.kind.trim().is_empty() {
+            return Err(serde::de::Error::custom(
+                "expected a nonempty credential type",
+            ));
+        }
+        Ok(())
+    }
+}
+
+async fn validate_adc_file(file: File) -> std::io::Result<()> {
+    let file = file.into_std().await;
+    tokio::task::spawn_blocking(move || {
+        // Stream past type-specific fields without retaining the credential contents.
+        let mut json = serde_json::Deserializer::from_reader(BufReader::new(file));
+        json.deserialize_map(AdcObject)?;
+        json.end()?;
+        Ok(())
+    })
+    .await?
+}
+
+async fn verify_credentials(
+    configured: ModelProviderInfo,
+    cwd: &AbsolutePathBuf,
+) -> Result<(), JSONRPCErrorError> {
+    configured
+        .validate_gcp_credential_file()
+        .map_err(|err| AuthErrorReason::CredentialFileInvalid.error(err))?;
+    let default_gcloud = configured.auth.is_none();
+    let mut candidate = ModelProviderInfo::create_gcp_provider();
+    merge_gcp_provider_overrides(Some(&mut candidate), configured).map_err(invalid_request)?;
+    apply_default_gcp_auth(&mut candidate, cwd);
+    candidate.validate().map_err(invalid_request)?;
+    let auth_file = candidate.credential_file.clone().or_else(|| {
+        if default_gcloud {
+            std::env::var_os("GOOGLE_APPLICATION_CREDENTIALS").map(PathBuf::from)
+        } else {
+            None
+        }
+    });
+    if let Some(path) = auth_file {
+        let path = cwd.as_path().join(path);
+        let file = open_credential_file(&path).await.map_err(|err| {
+            AuthErrorReason::CredentialFileInvalid.error(format!(
+                "Cannot read the Google Cloud credential file: {err}"
+            ))
+        })?;
+        if default_gcloud {
+            validate_adc_file(file).await.map_err(|_| AuthErrorReason::CredentialFileInvalid.error(
+                "Expected a Google Cloud credential file containing a JSON object with a nonempty string type.",
+            ))?;
+        }
+    }
+    let provider = create_model_provider(candidate, /*auth_manager*/ None);
+    provider
+        .runtime_base_url()
+        .await
+        .map_err(|err| invalid_request(err.to_string()))?;
+    // This fresh provider owns a command-backed manager. A refresh runs that command
+    // once and preserves errors that the inference auth path reduces to missing auth.
+    let auth_manager = provider
+        .auth_manager()
+        .ok_or_else(|| internal_error("Missing GCP auth manager."))?;
+    if let Err(err) = auth_manager.refresh_token().await {
+        if default_gcloud
+            && let RefreshTokenError::Transient(err) = err
+            && err.kind() == ErrorKind::NotFound
+            && cwd.as_path().is_dir()
+        {
+            return Err(AuthErrorReason::GcloudNotInstalled.error(
+                "Cannot find gcloud on the app-server host. Check its installation and PATH.",
+            ));
+        }
+        return Err(AuthErrorReason::AuthCommandFailed
+            .error("The Google Cloud auth command could not obtain an access token."));
+    }
+
+    Ok(())
+}
diff --git a/codex-rs/app-server/src/request_processors/bedrock_auth.rs b/codex-rs/app-server/src/request_processors/bedrock_auth.rs
index 9aba8f9c52..ce2183e718 100644
--- a/codex-rs/app-server/src/request_processors/bedrock_auth.rs
+++ b/codex-rs/app-server/src/request_processors/bedrock_auth.rs
@@ -1,5 +1,6 @@
 use super::config_processor::map_error as map_config_error;
 use crate::config_manager::ConfigManager;
+use crate::config_manager_service::PreparedProviderWrite;
 use crate::error_code::internal_error;
 use crate::error_code::invalid_request;
 use codex_app_server_protocol::ConfigBatchWriteParams;
@@ -7,9 +8,6 @@ use codex_app_server_protocol::ConfigEdit;
 use codex_app_server_protocol::ConfigWriteErrorCode;
 use codex_app_server_protocol::JSONRPCErrorError;
 use codex_app_server_protocol::MergeStrategy;
-use codex_config::CONFIG_TOML_FILE;
-use codex_config::ConfigLayerSource;
-use codex_config::format_config_layer_source;
 use codex_core::config::Config;
 use codex_core::config::edit::ConfigEditsBuilder;
 use codex_model_provider::AMAZON_BEDROCK_PROVIDER_ID;
@@ -19,100 +17,30 @@ pub(super) struct BedrockProviderConfig<'a> {
     pub(super) profile: Option<&'a str>,
 }
 
-pub(super) async fn configure_bedrock_provider(
+pub(super) async fn prepare_bedrock_provider(
     config_manager: &ConfigManager,
     config: BedrockProviderConfig<'_>,
-) -> Result<(), JSONRPCErrorError> {
-    let mut edits = vec![
-        (
-            "model_provider",
-            serde_json::json!(AMAZON_BEDROCK_PROVIDER_ID),
-        ),
-        (
-            "model_providers.amazon-bedrock.aws.profile",
-            serde_json::json!(config.profile),
-        ),
-    ];
-    if let Some(region) = config.region {
-        edits.push((
-            "model_providers.amazon-bedrock.aws.region",
-            serde_json::json!(region),
-        ));
-    }
-
-    let response = config_manager
-        .batch_write(ConfigBatchWriteParams {
-            edits: edits
-                .into_iter()
-                .map(|(key_path, value)| ConfigEdit {
-                    key_path: key_path.to_string(),
-                    value,
-                    merge_strategy: MergeStrategy::Replace,
-                })
-                .collect(),
-            file_path: None,
-            expected_version: None,
-            reload_user_config: false,
-        })
-        .await
-        .map_err(map_config_error)?;
-    if let Some(overridden) = response.overridden_metadata {
-        let message = overridden.message;
-        return Err(invalid_request(format!(
-            "Amazon Bedrock configuration cannot take effect: {message}"
-        )));
-    }
-    Ok(())
-}
-
-pub(super) async fn ensure_user_model_provider_can_be_bedrock(
-    config_manager: &ConfigManager,
-) -> Result<(), JSONRPCErrorError> {
+) -> Result<PreparedProviderWrite, JSONRPCErrorError> {
     let layers = config_manager
         .load_config_layers(/*cwd*/ None)
         .await
         .map_err(|err| internal_error(format!("failed to load configuration layers: {err}")))?;
-    let user_precedence = match layers.get_active_user_layer() {
-        Some(layer) => layer.name.precedence(),
-        None => ConfigLayerSource::User {
-            file: config_manager.user_config_path().map_err(|err| {
-                internal_error(format!("failed to resolve user config path: {err}"))
-            })?,
-            profile: None,
-        }
-        .precedence(),
-    };
-    if let Some((overriding_layer, effective_provider)) = layers
-        .layers_high_to_low()
-        .filter(|layer| layer.name.precedence() > user_precedence)
-        .find_map(|layer| {
-            layer
-                .config
-                .get("model_provider")
-                .map(|value| (layer, value))
-        })
-        && effective_provider.as_str() != Some(AMAZON_BEDROCK_PROVIDER_ID)
-    {
-        let source = format_config_layer_source(&overriding_layer.name, CONFIG_TOML_FILE);
-        return Err(invalid_request(format!(
-            "Amazon Bedrock login cannot select `{AMAZON_BEDROCK_PROVIDER_ID}` because {source} sets `model_provider` to {effective_provider}"
-        )));
+    let mut settings = vec![("aws.profile", config.profile)];
+    if let Some(region) = config.region {
+        settings.push(("aws.region", Some(region)));
     }
-
-    if layers
-        .effective_config()
-        .get("model_providers")
-        .and_then(|providers| providers.get(AMAZON_BEDROCK_PROVIDER_ID))
-        .and_then(|provider| provider.get("aws"))
-        .and_then(|aws| aws.get("credential_export"))
-        .is_some()
+    let (write, provider) = config_manager
+        .prepare_provider_config(&layers, AMAZON_BEDROCK_PROVIDER_ID, settings)
+        .map_err(map_config_error)?;
+    if provider
+        .aws
+        .is_some_and(|aws| aws.credential_export.is_some())
     {
         return Err(invalid_request(
             "Amazon Bedrock is configured to use `aws.credential_export`. Please clear this setting to use another sign-in method.",
         ));
     }
-
-    Ok(())
+    Ok(write)
 }
 
 pub(super) async fn clear_user_model_provider_if_bedrock(
diff --git a/codex-rs/app-server/tests/suite/v2/account.rs b/codex-rs/app-server/tests/suite/v2/account.rs
index 3ffcf0d2f7..259438bee8 100644
--- a/codex-rs/app-server/tests/suite/v2/account.rs
+++ b/codex-rs/app-server/tests/suite/v2/account.rs
@@ -1441,7 +1441,7 @@ async fn login_amazon_bedrock_rejects_non_bedrock_provider_override_without_chan
     .await??;
     assert_eq!(
         error.error.message,
-        "Amazon Bedrock login cannot select `amazon-bedrock` because session-flags sets `model_provider` to \"mock_provider\""
+        "Cannot configure `model_provider`: Overridden by session flags"
     );
     assert_eq!(load_file_auth(codex_home.path())?, expected_auth);
     assert_eq!(read_config_toml(codex_home.path())?, expected_config);
@@ -1470,10 +1470,9 @@ async fn login_amazon_bedrock_rejects_non_bedrock_provider_override_without_chan
 
 #[tokio::test]
 async fn login_amazon_bedrock_access_keys_rejects_overridden_aws_configuration() -> Result<()> {
-    for config_override in [
-        r#"model_providers.amazon-bedrock.aws.profile="other-account""#,
-        r#"model_providers.amazon-bedrock.aws.region="eu-west-1""#,
-    ] {
+    for (field, value) in [("profile", "other-account"), ("region", "eu-west-1")] {
+        let key = format!("model_providers.amazon-bedrock.aws.{field}");
+        let config_override = format!(r#"{key}="{value}""#);
         let codex_home = TempDir::new()?;
         create_config_toml(codex_home.path(), CreateConfigTomlParams::default())?;
         login_with_api_key(
@@ -1488,9 +1487,11 @@ async fn login_amazon_bedrock_access_keys_rejects_overridden_aws_configuration()
             .with_codex_home(codex_home.path())
             .without_auto_env()
             .with_env_overrides(&[("OPENAI_API_KEY", None)])
-            .with_args(&["-c", config_override])
+            .with_args(&["-c", &config_override])
             .build_initialized_with_timeout(DEFAULT_READ_TIMEOUT)
             .await?;
+        let config_path = codex_home.path().join("config.toml");
+        let expected_config = std::fs::read_to_string(&config_path)?;
 
         let request_id = mcp
             .send_login_account_request(json!({
@@ -1508,9 +1509,10 @@ async fn login_amazon_bedrock_access_keys_rejects_overridden_aws_configuration()
 
         assert_eq!(
             error.error.message,
-            "Amazon Bedrock configuration cannot take effect: Overridden by session flags"
+            format!("Cannot configure `{key}`: Overridden by session flags")
         );
         assert_eq!(load_file_auth(codex_home.path())?, expected_auth);
+        assert_eq!(std::fs::read_to_string(&config_path)?, expected_config);
     }
 
     Ok(())
diff --git a/codex-rs/app-server/tests/suite/v2/bedrock_setup.rs b/codex-rs/app-server/tests/suite/v2/bedrock_setup.rs
index 21ce43474b..3b0695c366 100644
--- a/codex-rs/app-server/tests/suite/v2/bedrock_setup.rs
+++ b/codex-rs/app-server/tests/suite/v2/bedrock_setup.rs
@@ -199,7 +199,11 @@ async fn setup_bedrock_profile_and_environment() -> Result<()> {
 
     let managed_home = TempDir::new()?;
     let managed_config_path = managed_home.path().join("config.toml");
-    std::fs::write(&managed_config_path, "")?;
+    std::fs::write(&managed_config_path, "model_provider = 'openai'")?;
+    std::fs::write(
+        managed_home.path().join("requirements.toml"),
+        "model_provider = 'amazon-bedrock'",
+    )?;
     login_with_bedrock_api_key(
         managed_home.path(),
         "managed-bedrock-api-key",
@@ -223,7 +227,6 @@ async fn setup_bedrock_profile_and_environment() -> Result<()> {
         .as_table_mut()
         .expect("config should be a table")
         .extend(toml::toml! {
-            model_provider = "amazon-bedrock"
             [model_providers.amazon-bedrock.aws]
             profile = "engineering"
             region = "us-west-2"
diff --git a/codex-rs/app-server/tests/suite/v2/gcp_setup_tests.rs b/codex-rs/app-server/tests/suite/v2/gcp_setup_tests.rs
new file mode 100644
index 0000000000..6be43f95c7
--- /dev/null
+++ b/codex-rs/app-server/tests/suite/v2/gcp_setup_tests.rs
@@ -0,0 +1,350 @@
+//! GCP setup validates before saving and shares provider configuration with subsequent requests.
+
+use anyhow::Context;
+use anyhow::Result;
+use app_test_support::TestAppServer;
+use codex_app_server_protocol::ClientRequest;
+use codex_app_server_protocol::GcpSetupParams;
+use codex_app_server_protocol::GcpSetupResponse;
+use codex_app_server_protocol::JSONRPCErrorError;
+use codex_app_server_protocol::ModelListParams;
+use codex_app_server_protocol::ModelListResponse;
+use codex_app_server_protocol::RequestId;
+use codex_protocol::openai_models::ModelVisibility;
+use pretty_assertions::assert_eq;
+use serde_json::Value;
+use serde_json::json;
+use std::path::Path;
+use std::path::PathBuf;
+use std::time::Duration;
+use tempfile::TempDir;
+use tokio::time::timeout;
+
+const CLEARED_AUTH_ENV: &[(&str, Option<&str>)] = &[
+    ("OPENAI_API_KEY", None),
+    ("CODEX_API_KEY", None),
+    ("OPENAI_FEDERATION_RULE_ID", None),
+    ("OPENAI_IDENTITY_TOKEN_FILE", None),
+    ("OPENAI_WORKLOAD_IDENTITY_CONTEXT", None),
+];
+
+const READ_TIMEOUT: Duration = Duration::from_secs(/*secs*/ 60);
+
+fn write_fake_gcloud(home: &Path) -> Result<PathBuf> {
+    #[cfg(unix)]
+    let script = home.join("gcloud");
+    #[cfg(unix)]
+    codex_utils_cargo_bin::write_executable(
+        &script,
+        r#"#!/bin/sh
+if [ -f fail ]; then exit 1; fi
+printf '%s\n' "$GOOGLE_APPLICATION_CREDENTIALS" >> credential-log
+printf '%s\n' setup-token
+"#,
+    )?;
+    #[cfg(windows)]
+    let script = home.join("gcloud.cmd");
+    #[cfg(windows)]
+    std::fs::write(
+        &script,
+        "@echo off\r\nif exist fail exit /b 1\r\nsetlocal EnableDelayedExpansion\r\necho(!GOOGLE_APPLICATION_CREDENTIALS!>>credential-log\r\nendlocal\r\necho setup-token\r\n",
+    )?;
+    Ok(script)
+}
+
+async fn gcp_app_server(
+    home: &Path,
+    env_overrides: &[(&str, Option<&str>)],
+) -> Result<TestAppServer> {
+    let existing_path = std::env::var_os("PATH").unwrap_or_default();
+    let child_path = std::env::join_paths(
+        std::iter::once(home.to_path_buf()).chain(std::env::split_paths(&existing_path)),
+    )?;
+    TestAppServer::builder()
+        .with_codex_home(home)
+        .with_env_overrides(&[
+            (
+                "PATH",
+                Some(child_path.to_str().context("child PATH must be UTF-8")?),
+            ),
+            ("GOOGLE_APPLICATION_CREDENTIALS", None),
+            ("GOOGLE_CLOUD_PROJECT", Some("ambient-project")),
+            ("GOOGLE_CLOUD_LOCATION", Some("ambient-location")),
+        ])
+        .with_env_overrides(CLEARED_AUTH_ENV)
+        .with_env_overrides(env_overrides)
+        .build_initialized_with_timeout(READ_TIMEOUT)
+        .await
+}
+
+async fn setup_error(app_server: &mut TestAppServer, params: Value) -> Result<JSONRPCErrorError> {
+    let id = app_server
+        .send_raw_request("account/gcp/setup", Some(params))
+        .await?;
+    let error = timeout(
+        READ_TIMEOUT,
+        app_server.read_stream_until_error_message(RequestId::Integer(id)),
+    )
+    .await??;
+    assert_eq!(error.error.code, -32600);
+    Ok(error.error)
+}
+
+#[tokio::test]
+async fn setup_gcp_saves_overrides_restores_adc_and_refreshes_catalog() -> Result<()> {
+    let home = TempDir::new()?;
+    let home = home.path();
+    write_fake_gcloud(home)?;
+    let mut catalog = codex_models_manager::bundled_models_response()?;
+    catalog.models.retain(|model| model.slug == "gpt-6.1-sol");
+    let model = catalog
+        .models
+        .first_mut()
+        .context("bundled catalog must include a supported Vertex model")?;
+    model.visibility = ModelVisibility::List;
+    model.supported_in_api = true;
+    let mut unsupported_model = model.clone();
+    unsupported_model.slug = "test-non-vertex-model".to_string();
+    catalog.models.push(unsupported_model);
+    let catalog_path = home.join("models.json");
+    std::fs::write(&catalog_path, serde_json::to_vec(&catalog)?)?;
+    let config_path = home.join("config.toml");
+    let mut expected_config = json!({
+        "model_catalog_json": catalog_path,
+        "model_providers": {"gcp": {"http_headers": {"X-Existing": "preserved"}}}
+    });
+    std::fs::write(&config_path, toml::to_string(&expected_config)?)?;
+    let selected = home.join("selected.json");
+    let ambient = home.join("ambient.json");
+    for file in [&selected, &ambient] {
+        std::fs::write(file, r#"{"type":"external_account"}"#)?;
+    }
+    let mut app_server = gcp_app_server(
+        home,
+        &[(
+            "GOOGLE_APPLICATION_CREDENTIALS",
+            Some(ambient.to_str().context("non-UTF-8 ADC path")?),
+        )],
+    )
+    .await?;
+    let before: ModelListResponse = app_server
+        .request(|request_id| ClientRequest::ModelList {
+            request_id,
+            params: ModelListParams::default(),
+        })
+        .await?;
+    assert_eq!(before.data.len(), 2);
+
+    let _: GcpSetupResponse = app_server
+        .request(|request_id| ClientRequest::GcpSetup {
+            request_id,
+            params: GcpSetupParams {
+                project_id: " selected-project ".to_string(),
+                location: Some(" us-central1 ".to_string()),
+                credential_file: Some(selected.to_string_lossy().into_owned()),
+            },
+        })
+        .await?;
+    expected_config["model_provider"] = json!("gcp");
+    expected_config["model_providers"]["gcp"]["project_id"] = json!("selected-project");
+    expected_config["model_providers"]["gcp"]["location"] = json!("us-central1");
+    expected_config["model_providers"]["gcp"]["credential_file"] = json!(selected);
+    assert_eq!(
+        toml::from_str::<Value>(&std::fs::read_to_string(&config_path)?)?,
+        expected_config
+    );
+
+    let after: ModelListResponse = app_server
+        .request(|request_id| ClientRequest::ModelList {
+            request_id,
+            params: ModelListParams::default(),
+        })
+        .await?;
+    assert_eq!(after.data.len(), 1);
+    assert_eq!(after.data[0].model, "gpt-6.1-sol");
+
+    // A managed GCP selection takes precedence without replacing the user's preference.
+    expected_config["model_provider"] = json!("openai");
+    std::fs::write(&config_path, toml::to_string(&expected_config)?)?;
+    std::fs::write(home.join("requirements.toml"), "model_provider = 'gcp'")?;
+    let request_id = app_server
+        .send_raw_request(
+            "account/gcp/setup",
+            Some(json!({"projectId": "other-project"})),
+        )
+        .await?;
+    let _: GcpSetupResponse = app_server.read_response(request_id).await?;
+    expected_config["model_providers"]["gcp"]["project_id"] = json!("other-project");
+    expected_config["model_providers"]["gcp"]["location"] = json!("global");
+    expected_config["model_providers"]["gcp"]
+        .as_object_mut()
+        .unwrap()
+        .remove("credential_file");
+    assert_eq!(
+        toml::from_str::<Value>(&std::fs::read_to_string(&config_path)?)?,
+        expected_config
+    );
+    assert_eq!(
+        std::fs::read_to_string(home.join("credential-log"))?
+            .lines()
+            .map(Path::new)
+            .collect::<Vec<_>>(),
+        [selected.as_path(), ambient.as_path()]
+    );
+    Ok(())
+}
+
+#[tokio::test]
+async fn setup_gcp_rejects_invalid_inputs_and_credentials_before_saving() -> Result<()> {
+    let home = TempDir::new()?;
+    let home = home.path();
+    let config_path = home.join("config.toml");
+    std::fs::write(&config_path, "")?;
+    let invalid = home.join("invalid.json");
+    std::fs::write(&invalid, r#"["authorized_user"]"#)?;
+    let mut server = gcp_app_server(
+        home,
+        &[("PATH", Some(home.to_str().context("non-UTF-8 PATH")?))],
+    )
+    .await?;
+    let params = json!({"projectId": "project"});
+    for (request, reason) in [
+        (json!({"projectId": " "}), None),
+        (params.clone(), Some("gcloudNotInstalled")),
+        (
+            json!({"projectId": "project", "credentialFile": invalid}),
+            Some("credentialFileInvalid"),
+        ),
+    ] {
+        let error = setup_error(&mut server, request).await?;
+        assert_eq!(error.data, reason.map(|reason| json!({ "reason": reason })));
+    }
+
+    write_fake_gcloud(home)?;
+    std::fs::write(home.join("fail"), "")?;
+    let error = setup_error(&mut server, params).await?;
+    assert_eq!(error.data, Some(json!({"reason": "authCommandFailed"})));
+    assert_eq!(std::fs::read_to_string(config_path)?, "");
+    Ok(())
+}
+
+#[tokio::test]
+async fn setup_gcp_validates_the_auth_override_relative_to_the_startup_directory() -> Result<()> {
+    let home = TempDir::new()?;
+    let startup = TempDir::new()?;
+    let script = write_fake_gcloud(home.path())?;
+    let auth = toml::Value::try_from(json!({"command": script, "cwd": "."}))?;
+    let config_path = home.path().join("config.toml");
+    std::fs::write(&config_path, "")?;
+    // The same command succeeds in CODEX_HOME but fails in the catalog's startup context.
+    std::fs::write(startup.path().join("fail"), "")?;
+    let mut server = TestAppServer::builder()
+        .with_codex_home(startup.path())
+        .with_env_overrides(CLEARED_AUTH_ENV)
+        .with_env_overrides(&[(
+            "CODEX_HOME",
+            Some(home.path().to_str().context("non-UTF-8 CODEX_HOME")?),
+        )])
+        .with_args(&["-c", &format!("model_providers.gcp.auth={auth}")])
+        .build_initialized_with_timeout(READ_TIMEOUT)
+        .await?;
+
+    let error = setup_error(&mut server, json!({"projectId": "project"})).await?;
+    assert_eq!(error.data, Some(json!({"reason": "authCommandFailed"})));
+    assert_eq!(std::fs::read_to_string(config_path)?, "");
+    Ok(())
+}
+
+#[tokio::test]
+async fn setup_gcp_rejects_policy_and_endpoint_conflicts_before_saving() -> Result<()> {
+    for (file, policy, expected_message) in [
+        (
+            "managed_config.toml",
+            "forced_login_method = 'chatgpt'",
+            "Google Cloud setup is disabled. Use ChatGPT login instead.",
+        ),
+        (
+            "managed_config.toml",
+            "[model_providers.gcp]\nproject_id = 'managed-project'",
+            "Cannot configure `model_providers.gcp.project_id`: Overridden by legacy managed_config.toml",
+        ),
+        (
+            "managed_config.toml",
+            "[model_providers.gcp]\nbase_url = 'https://vertex.example.com/v1'",
+            "Google Cloud has a custom `base_url`",
+        ),
+        (
+            "requirements.toml",
+            "model_provider = 'openai'",
+            "requirements set `model_provider` to `openai`",
+        ),
+        (
+            "requirements.toml",
+            "[application.network.domains]\n'oauth2.googleapis.com' = 'allow'",
+            "disabled by application network restrictions",
+        ),
+    ] {
+        let home = TempDir::new()?;
+        let home = home.path();
+        write_fake_gcloud(home)?;
+        std::fs::write(home.join(file), policy)?;
+        let config_path = home.join("config.toml");
+        std::fs::write(&config_path, "")?;
+        let mut app_server = gcp_app_server(home, &[]).await?;
+        let error = setup_error(
+            &mut app_server,
+            json!({"projectId": "project", "credentialFile": home.join("missing.json")}),
+        )
+        .await?;
+        assert!(
+            error.message.contains(expected_message),
+            "expected {expected_message:?}, got {:?}",
+            error.message
+        );
+        assert_eq!(std::fs::read_to_string(config_path)?, "");
+        assert!(!home.join("credential-log").exists());
+    }
+    Ok(())
+}
+
+#[cfg(unix)]
+#[tokio::test]
+async fn setup_gcp_stops_the_credential_command_when_network_permission_is_revoked() -> Result<()> {
+    use core_test_support::process::wait_for_pid_file;
+    use core_test_support::process::wait_for_process_exit;
+
+    let home = TempDir::new()?;
+    let config_path = home.path().join("config.toml");
+    let initial_config = r#"
+[model_providers.gcp.auth]
+command = "sh"
+args = ["-c", "echo $$ > refresh.pid; exec sleep 120"]
+timeout_ms = 120000
+"#;
+    std::fs::write(&config_path, initial_config)?;
+    let mut app_server = gcp_app_server(home.path(), &[]).await?;
+    let request_id = app_server
+        .send_raw_request("account/gcp/setup", Some(json!({"projectId": "project"})))
+        .await?;
+    let pid = wait_for_pid_file(&home.path().join("refresh.pid")).await?;
+
+    std::fs::write(
+        home.path().join("requirements.toml"),
+        "[application.network]",
+    )?;
+    app_server
+        .send_raw_request("config/read", Some(json!({})))
+        .await?;
+    let error = timeout(
+        READ_TIMEOUT,
+        app_server.read_stream_until_error_message(RequestId::Integer(request_id)),
+    )
+    .await??;
+    assert_eq!(
+        error.error.message,
+        "application network permission was revoked"
+    );
+    wait_for_process_exit(&pid).await?;
+    assert_eq!(std::fs::read_to_string(config_path)?, initial_config);
+    Ok(())
+}
diff --git a/codex-rs/app-server/tests/suite/v2/mod.rs b/codex-rs/app-server/tests/suite/v2/mod.rs
index 42c6ecb14f..032f5e5a32 100644
--- a/codex-rs/app-server/tests/suite/v2/mod.rs
+++ b/codex-rs/app-server/tests/suite/v2/mod.rs
@@ -76,6 +76,8 @@ mod frozen_cloud_skills;
 mod fs;
 #[path = "gateway_oauth_tests.rs"]
 mod gateway_oauth;
+#[path = "gcp_setup_tests.rs"]
+mod gcp_setup;
 mod git_attribution;
 mod guardian_v2;
 mod history_notes_extension;
diff --git a/codex-rs/login/src/auth/external_bearer.rs b/codex-rs/login/src/auth/external_bearer.rs
index ed58f4ccdb..4fcea04f0e 100644
--- a/codex-rs/login/src/auth/external_bearer.rs
+++ b/codex-rs/login/src/auth/external_bearer.rs
@@ -130,10 +130,13 @@ async fn run_provider_auth_command(
             ))
         })?
         .map_err(|err| {
-            io::Error::other(format!(
-                "provider auth command `{}` failed to start: {err}",
-                config.command
-            ))
+            io::Error::new(
+                err.kind(),
+                format!(
+                    "provider auth command `{}` failed to start: {err}",
+                    config.command
+                ),
+            )
         })?;
 
     if !output.status.success() {
diff --git a/codex-rs/model-provider-info/src/lib.rs b/codex-rs/model-provider-info/src/lib.rs
index 9678d7056b..78fc793c35 100644
--- a/codex-rs/model-provider-info/src/lib.rs
+++ b/codex-rs/model-provider-info/src/lib.rs
@@ -800,40 +800,7 @@ pub fn merge_configured_model_providers(
                 }
             }
         } else if key == GCP_PROVIDER_ID {
-            provider
-                .validate_gcp_credential_file()
-                .and_then(|()| provider.validate())
-                .map_err(|message| format!("model_providers.{GCP_PROVIDER_ID} {message}"))?;
-            let base_url_override = provider.base_url.take();
-            let auth_override = provider.auth.take();
-            let credential_file_override = provider.credential_file.take();
-            let http_headers_override = provider.http_headers.take();
-            let project_id_override = provider.project_id.take();
-            let location_override = provider.location.take();
-            if provider != ModelProviderInfo::default() {
-                return Err(format!(
-                    "model_providers.{GCP_PROVIDER_ID} only supports changing \
-`base_url`, `auth`, `http_headers`, `project_id`, `location`, and `credential_file`; other non-default provider fields are not supported"
-                ));
-            }
-
-            if let Some(built_in_provider) = model_providers.get_mut(GCP_PROVIDER_ID) {
-                built_in_provider.project_id = project_id_override;
-                built_in_provider.location = location_override;
-                built_in_provider.credential_file = credential_file_override;
-                if let Some(base_url_override) = base_url_override {
-                    built_in_provider.base_url = Some(base_url_override);
-                }
-                if let Some(auth_override) = auth_override {
-                    built_in_provider.auth = Some(auth_override);
-                }
-                if let Some(http_headers_override) = http_headers_override {
-                    built_in_provider
-                        .http_headers
-                        .get_or_insert_default()
-                        .extend(http_headers_override);
-                }
-            }
+            merge_gcp_provider_overrides(model_providers.get_mut(&key), provider)?;
         } else {
             model_providers.entry(key).or_insert(provider);
         }
@@ -842,6 +809,48 @@ pub fn merge_configured_model_providers(
     Ok(model_providers)
 }
 
+/// Validates GCP overrides and applies them when the built-in provider is present.
+pub fn merge_gcp_provider_overrides(
+    built_in: Option<&mut ModelProviderInfo>,
+    mut overrides: ModelProviderInfo,
+) -> Result<(), String> {
+    overrides
+        .validate_gcp_credential_file()
+        .and_then(|()| overrides.validate())
+        .map_err(|message| format!("model_providers.{GCP_PROVIDER_ID} {message}"))?;
+    let base_url = overrides.base_url.take();
+    let auth = overrides.auth.take();
+    let credential_file = overrides.credential_file.take();
+    let http_headers = overrides.http_headers.take();
+    let project_id = overrides.project_id.take();
+    let location = overrides.location.take();
+    if overrides != ModelProviderInfo::default() {
+        return Err(format!(
+            "model_providers.{GCP_PROVIDER_ID} only supports changing \
+`base_url`, `auth`, `http_headers`, `project_id`, `location`, and `credential_file`; other non-default provider fields are not supported"
+        ));
+    }
+
+    if let Some(provider) = built_in {
+        provider.project_id = project_id;
+        provider.location = location;
+        provider.credential_file = credential_file;
+        if let Some(base_url) = base_url {
+            provider.base_url = Some(base_url);
+        }
+        if let Some(auth) = auth {
+            provider.auth = Some(auth);
+        }
+        if let Some(http_headers) = http_headers {
+            provider
+                .http_headers
+                .get_or_insert_default()
+                .extend(http_headers);
+        }
+    }
+    Ok(())
+}
+
 pub fn create_oss_provider(default_provider_port: u16, wire_api: WireApi) -> ModelProviderInfo {
     // These CODEX_OSS_ environment variables are experimental: we may
     // switch to reading values from config.toml instead.